Direct reduction of iron oxide and other metallic oxides may be conducted in rotary hearth furnaces ("RHF") using pelletized or briquetted feed deposited upon the rotating hearth.
Briefly, an RHF is a continuous reheating furnace generally having a circular inner wall circumscribed by a spaced circular inner & outerwall. The void formed therebetween includes a circular rotating hearth. In order to retain the heat generated with the furnace the walls are relatively low so as to enable the roof to be close to the hearth. Burners may be installed in the inner and outer walls and in the roof.
Material is usually loaded (dropped) onto the rotating hearth by a conveyor or chute. After the material is carried on the hearth, it is usually removed by a discharge screw. Due to high temperatures (1300.degree.-2300.degree. F. [704.degree.-1260.degree. C.]) involved, the screw is frequently water cooled. See U.S. Pat. No. 3,443,931. Gases are permitted to vent through a flue located in the roof.
A conventional conveying or discharge screw consists of a central shaft with a solid helical flight welded thereto. A cooling fluid is passed through a bore disposed within the shaft. Other screw designs utilize a plurality of spaced solid flights disposed about the shaft.
Due to corrosive nature of the gases and materials present within the RHF, coupled with the high temperatures therein, the discharge screw is subject to frequent failure. The flights generally deteriorate. Corrosion and erosion caused by high temperatures and bad actors (sodium, sulfides, chlorides, fluorides, potassium lead, zinc, tin, iron, nickel and chromium) within the RHF oftentimes chew up the screws and render them useless after only about three months. Expensive materials such as HH alloy (20% nickel, 20% chromium, remainder iron) as well as IN 659 were not satisfactory.
In addition, the spaces between the flights accumulate fluffy fines which tend to cake together. The fines act as a sponge which serve to collect and concentrate the corrosive gases present within the furnace.
As can be imagined, frequent screw replacement necessitates frequent downtime, high maintenance and labor costs, and inefficient use of the furnace which in turn lead to higher unit costs. Clearly a better screw design is necessary.